Flame resistant polyimide-coated conductor having a linear polyimide layer covered by an aromatic polyamide

ABSTRACT

Laminates primarily for electrical insulation comprise at least one lamina of a thermoplastic polyimide, e.g., poly(1, 12dodecamethylene pyromellitimide) and a covering lamina of an infusible or high-melting polymer, especially an aromatic polyimide. An insulated airframe wire is described in detail.

227,252; 117/72, 128.4, 132 B, 132C, 16], 218; 260/47,75; 174/1 10, 120SR, 121 SR I Unlted States Patent 1111 3,607,387

[72] inventors Vincent L. Lanu; [56] References Cited Edward C. Stivers,both of Atherton, Calif. n- STATES PATENTS P 760513 3,242,128 3/1966Chalmers 260 326 [221 PM 1968 3 271 366 9/1966 KI'CUZ 260/47 [451Patented 1971 3 316 212 4/1967 An 81 260/47 hem Cor ration g o [731Assgnee k 3,326,863 6/1967 Tatum 6181.. 260/78.4 3,342,768 9/1967Chalmers 260/326 Primary Examiner.l0hn T. Goolkasian Assistant Examingrwiuard E Hoag CONDUCTOR HAVING A LINEAR POLYIMIDE Ammey Lyon and LyonLAYER COVERED BY AN AROMATIC POLYAMIDE 9 Claims, 2 Drawing Figs.

[52] U.S.Cl 117/218, 117/75,117/72,l17/232,l17/128.4,117/161,

161/214, 161/227, 161/252, 174/120 ABSTRACT: Laminates primarily forelectrical insulation [51] Int.Cl H0lb 7/02, comprise at least onelamina of a thermoplastic polyimide, HOlb 3/00 e.g., poly( l,l2-dodecamethylenc pyromellitimide) and a [50] Field of Search 161/214,covering lamina of an infusible or high-melting polymer, especially anaromatic polyimide. An insulated airframe wire is described in detail.

'FLAMEIRESISTANT:POLYIMIDE+COATED1CONDUCTOR HAVING AIZINEA'RTPOLYIMIDEIJAYER'COVERED- BY AN AROM ATIGPODYAMIDE BACKGROUNDTHE INVENTION This-invention relates to ,structuresfformedatleast inpart from polyimides, and toprocess'fortheir manufacture.

In copending application Ser. No. 760,659 filed of even date herewith,entitled Plymers,EProcess and.Articles,the

invention of Edward C. Stive'rs,'thedisclosureof which is incorporatedby reference herein, there are described'certain thermoplastically'processable polymers, more specifically, poly(. l l 2-dodecamethylei1epyromellitimide) and poly( 1,13

tridecamethylenepyromellitimide). Asstated in'that applica- :tion,these-polymers may beextruded into tubing, sheets'and processable,polyimides meet many purely electrical .requirements, theinsulation'hasto be. capable of meetingincreasingly stringent nonelectricalitests toensureits safety and reliability under severe conditions. Moreover, manyof these require- .mentsare to someextent conflicting, in that aphysicalpropertyessential-tomeet one'test isi'a disadvantage-when:thematerial issubjected ,to another. In many wire and cable applica- -tions,:forexample, itis;necessary thatvthe insulation beiflexible sothat the-wirecan begivenvarious'configurations. ?It is also -necessary, howev,er,that the insulation-have aflow notch sen- :sitivity, i.e.,-that.a cutinthe surface =ofrthe insulation .does not propagate through to theconductor. 'fNotch sensitivity decreases withtheelastic modulus-ofamaterial, but-materials with .a high moduliare not flexible except inathinlayers.

In .many applications,;forexample, in insulation for aircraft, wherefueL lubricantsand hydraulic fluids are-present, the insulation must beunaffected by prolonged'exposure 'to such materials. Further,.as;operating temperatures of electrical and electronic equipment increase,improved resistance to heat- .aging is required; i.e., the abilitytooperate for prolonged periods .at elevated temperatures is necessary.A further requirement is resistance to tire. Although it is possible toincorporate flame retardants inv the insulating materials, suchadditives often have deleterious effects on other properties of thematerial, especially the physical and electrical properties, and theymay cause corrosion of the electrical components to be insulated. It haspreviously been proposed to meet these SUMMARY OF THE INVENTION It is anobject of the present invention to provide a laminate which combines theproperties of low density and good electrical properties with resistancetofire, mechanical abuse and chemical attack. The present inventionprovides a laminate comprising at least one lamina of athermoplastically processable polyimide and at least one lamina of anintractable polymer, for example an aromatic polyimide. The laminate.may contain laminae of other materials, and the laminate may be appliedto or in contact with a substrate, for example an electrical componentto be insulated. The V invention accordingly also provides an electricalcomponent, especially an electrical conductor,*having an insulationcomprising a thermoplasticallyprocessable polyimide lamina and anintractable polymer lamina. The laminae may, of course, be formed ofcompositions comprising the above-specified polymers in admixture withother materials,for example, blendswith other polymers, and compositions.consisting essentially of the specified polymer with theusualadditives, and the:polymers may be cross-linked where appropriate.

The term aromatic polyimide relates to those polymers derived frompredominately aromatic starting materials, as exemplified in U.S. Pat.No. 3,179,630 to 3,179,634, such materials being generally not amendable.to'processing, and being formed in situ from a processable precursor,e.g., a

polyamic acid..Examples of the manufacture and conversion of such acidsare given US. Pat. No. 3,179,614 assigned, as are'No. 3,179,630t03,179,634 to E. l. du 'Pont de'Nemours .andCompany, the disclosuresofallwhich patentsare incorporated herein by reference.

.DESCRIPTIONQOF THE DRAWINGS *FIG. 1 is a schematicview of a conductorinsulated according to the present invention.

FIG. 2 is aschematic view of aconductor insulated accordingto analternate embodiment of thepresent invention.

DESCRIPTION OF THE'PREFERR'ED EMBODIMENTS A preferred embodiment of thepresent invention is a laminate for electrical insulation comprising.two or more laminae of a thermoplastically processable polyimide, oneor more laminae of an aromatic polyimide and optionally a lamina of apigment-comprising thermoplastically processable polyimide. The abovespecified laminae are listed in the preferred order,1beginning with thatnearest to the'body to be insulated.

Clearly, the dimensions of the various laminae will vary with theintended .use of the laminate, as will the relative dimensions of thelaminae. It will be obvious to those skilled in the art that it is amatter of simple experiment to determine dimensions and ratios ofdimensions suitable for insulating a wire of a particular gauge; thedimensions will depend not only on the dimensions of .the wire butalsothe number of wires in the cable, the electrical stresses 'to which thewire is to be subjected in normal use 'and in overload, the physicalmaltreatment likely, the flexibility and weight requirements, etc.

In general, however, it has been found that the processable polyimidemay suitably form from about 65 to 95 percent of the wall thickness,measured radially, of a given wire insulation, preferably 75 to percentof the thickness. In general, a total thickness of at least about 0.0005inch, preferably at least 0.001 inch, of aromatic polyimide lamina orlaminae should be present to reduce the access of oxygen to anacceptable level, to achieve flame-resistance and good heat-agmg.

The laminate may be formed by casting;e'xtrusion, coating, deposition orany other technique suitable for forming layers of the materialsconcerned. The processable polyimide may be extruded, for example in theform of a tube, and further laminae (either of the processable polyimideitself or of a different thermoplastically processable polymer) may beextruded over the first material. Alternatively, two or more laminae maybe simultaneously extruded. Laminae of materials such as the aromaticpolyimide which cannot be extruded or otherwise thermoplasticallyprocessed may be applied to the surface of a previously formed laminaby, for example, brush-coating or spraying with or immersion in asolution of dispersion of a precursor of the material or a dispersion ofthe material itself. For example, an aromatic polyimide may be appliedto the surface of a lamina in the form of a solution or dispersion ofits precursor, the corresponding polyamic acid. This can then be heatedto convert it to lamina of the polyimide. The smooth finish obtainablewith an outer lamina which is applied by coating provides a productwhich has a good appearance and improved handling properties.

The suitability of a particular polymer for use as the intractablepolymer is determined by physical properties rather than by the chemicalstructure of the polymer. It is therefore to be understood that theinvention is not limited to the use of aromatic polyimides as theintractable polymer. Such polyimides have the physical propertiesnecessary for this invention, and are to be regarded as illustrativeexamples of suitable materials. These polyimides do, however, have theadditional advantage that in the form of the polyamic acid precursorthey have very good compatibility as shown by good adhesion, with thethermoplastic polyimide, being soluble in the same solvents. Suchcompatibility is of course a desirable property of. the intractablepolymer at least in the form in which it is applied.

Other desirable properties include nonfiammability, high meltingtemperature (at least that of the thermoplastic polyimide and preferablyabove the pyrolysis temperature of the thermoplastic polyimide) or,preferably, infusibility, high-temperature oxidation resistance (so asto give the product a life of at least 200 hours at 200C), high solventresistance, an elongation at room temperature of at least 50 percent, atleast in a layer of the thickness used, high elastic modulus, and highcut-through resistance.

The invention is further illustrated by reference to the drawings. Fig.1 shows electrical conductor 1 which is insulated by an outer coat 2 ofa processable polyimide and an inner coat 3 of an infusible aromaticpolyimide. If desired,

there may be two or more inner coats of the processable polyi- V EXAMPLE1 A 20 AWG stranded (19/32) silver plated copper conductor having anoutside diameter of 0.040 inch was insulated with a compositionconsisting of Parts by weight Poly (l,l2 -dodecamethylencpyromellitimide),

inherent viscosity 1.2

lrganox I010 (pentaerythrityltetra-3,5-ditcrt-butyl-4- hydroxydihydrocinnamatc) Trinonyl phenyl phosphitc The inherent viscosityvalues given in this specification were measured in m-cresol at 70 C.,at a concentration of 0.5 g./ 100 ml. The composition was in the form ofa powder blend, and was applied to the conductor by tubing from a 1.5inch Sterling 24 to l length/diameter vented extruder operating attemperature of 305 C. (rear) to 330 C. (front) in five zones under avacuum of approximately 28 inch Hg, the wire being preheated to about315 C. An insulation of approximately 0.004 inch wall thickness wasapplied, resulting in an outside diameter of 0.048 to 0.049 inch. Asecond layer of the same composition was then extruded over theinsulated conductor preheated to about 180 C. to give an overall outsidediameter of 0.057 to 0.058 inch, using the same temperature profile asin the first extrusion.

The insulated wire was then coated with a layer of the polymer derivedfrom oxydianiline and pyromellitic dianhydride, i.e., poly(4,4-phenylether pyromellitimide). This polymer, supplied in the form of a 17percent solids content solution of the polyamic acid precursor inN-methylpyrrolidone by E. I. du Pont de Nemours and Company under thedesignation RC-5044, was applied as the precursor from a bath of thesolution which had been concentrated to a 25 percent solids content bypartial evaporation of solvent. The

"coated wire was passed at 3 feet per minute from the bath up- 7 EXAMPLE2 A sample wire as obtained in example 1 was irradiated by high-energyelectrons from a l Mev. General Electric resonant transformer to a doseof .80 mr. The Youngs modulus of the wire at 325 C. in nitrogen was 25p.s.i., and the gel content of the cross-linkedpoly(l,l2-dodecamethylene pyromellitimide) measured inrefluxing'm-cresol was approximately percent. 7

EXAMPLE 3 The procedure of example I was repeated, except that only twolayers of aromatic polyimide were applied to the conduc tor insulated,with the thermoplastic polyimide. The coated conductor was then further'coated with a solution of the polyamic acid precursor of thethermoplastic polyimide. This was a 37 percent solids content solutionof the corresponding polyamic acid in dimethylacetamide containing 2percent (based on polymer) of lrganox 1010 and 25 percent (based onpolymer) of pigment-grade titanium dioxide (E. l. du Pont de NemoursTiPure R-900). The latter was incorporated into the solution by blendingon a 3-roll paint mill at room temperature and degassing.

The coating was cured in the manner described in example 1, the outerlayer being 0.0005 inch thick, giving an insulated wire if a finaldiameter of 0.061 inch, white in color, and readily markable.

EXAMPLE 4 A 20 AWG stranded (19/32) silver-plated copper conductor wasinsulated by the following composition:

Parts by Weight Poly (l,l2-dodecamethylene pyromellitimide), inherentviscosity 1.6 lrganox 1010 2 EXAMPLE 5 Lengths of wire as described inexample 4 were hung over an 0.75 inch diameter rod, each end of eachwire being weighted with a load of 2 pounds, as were samples of similarwire insulated with 0.0085 of poly(1,12-dodecamethylene pyromellitimide)but without the aromatic polyimide coating. All the samples were placedin a forced air oven at 200 C., for the periods specified in the tablebelow. After the specified period, the samples were removed and theportion of the wire which was around the rod immersed in a salt solutionat room temperature. An increasing voltage was then applied to theconductor, and the voltage at which insulation broke down noted. Thesamples were also examined, on removal from the oven, for cracks in theinsulation at the end of the insulated portion and at the portion bentaround the rod.

Unlaminated Polymer Laminate Wire Types B polycthylcnelpolyvinylidcncfluoride Days in Oven Breakdown Voltage Breakdown Voltage C l'luorinatedethylene propylene (Kv) (Kv) copolmerl 5 polyvinylidene fluoride Dpolyvinyl chloridclpolyvinylidcne 6 l6 niwiiiic 7 l3 E fluorinatedethylene propylene 8 12 crack! at end sample l3 eopolymer/ 9 cracked atbent portion l l aromatic polyimide l cracked ilt bent portion WIRE TYPEIt will be seen that the provision of a layer of aromatic polyi-Property or Test A u c u E mide increases the life of the insulationfrom 7 to 9 days at 200 C. it is believed that this is achieved bylimiting the acl Weight cess of oxygen to the lamina of thethermoplastic polyimide. x f I) 2 3 4 5 l Samples of the same twomaterials were tested for flammabiligx' f a n 2 5 3 4 ty by the testspecified in the US. Military Specification MlL- Tape Abrasion i 2 i 2 45 W-8l044 (referred to as the 60 test) and in a vertical test. In Smilefi t') I 2 4 3 5 the latter test, a sample insulated wire is hungvertically and :J2ysnam)c(g t-thru l 3 4 4 z the tip of a 1.5-inch highflame is contacted with the bottom of Dymnhc Cum," 2 3 3 1 theinsulation for 12 seconds. The flame is then removed and 2011f the timeof burning noted.

C. l 3 s 4 2 Solvent Resistance l 3 3 5 l 25 Current Overload I 3 4 5 lA :5 Amp of Current Overload l 2 4 5 3 Unlaminated Polymer Laminate 40Amp I Notes on Tests l. 2 lb.l400 grit tape i Bums cnmplcely 3-4 8950 5223713?z iiichlminute 4. 0.005 inch "31,90" Wedge L5 5. Hydraulic FluidSkydroV' inchelol'polymerflowed dnvvnintothc Home All wires listedpassed tests on insulation resistance, cold bend (at 65 C.),flammability (60 and vertical) and notch sensitivity. In thequantitative tests listed, the laminate of the present invention has thehighest overall performance, in- Thus' thipmvlslon of the thm layeraromzftlc polylmlde dicating that the combination of thethermoplastically v'ery effectwely renders the Polymer msulatmnprocessable polyimide and an intractable polymer provides an small! 40outstanding electrical insulation.

We claim: EXAMPLE 6 1. An insulated electrical conductor, the insulationcomprising an inside coat of poly(l,l2-dodecamethylene pyromellitimide)and an outside coat of an infusible aromatic polyimide. z g zx z s i z i2. An insulated wire, the insulation comprising two coats of 8 ran 5w're) a y amma e 0 e poly(1,12-dodecamethylene pyromellitimide)underlying at present f prepalied, as j' f m example were least twocoats of an infusible aromatic polyimide. compared wlth those of slmllarwire Insulated by a number of 3 An insulated wire as claimed in claim 2having an addi materials presently used or proposed for use asinsulation for tiona] outermost coat comprising pigmented POM 1 airframewire. Although the dimensions of the various msudodecamethylenepyromellitimide) lated wires differ, all wires tested were of the sizeselected for A wire as claimed in claim 3 wherein the aromatic po]yi thestated purpose-general use in cables connecting electrlc mide ispoly(4,4i phenyl ethar pyromellitimide) and electronic components inaircraft. in all materials, there 5 An m d wifg h insulation comprisingat least two are two polymers, lndlcat ng, as stated above, that h coatsconsisting essentially of poly(1,12-dodecamethylene fllctlngrequirement-5 have necessllated a cqmbmallon P pyromellitimide)underlying at least two coats consisting espertles. ln wires B, C, andD, the first material forms the inner ti ll f ol (4,4-phen l etherpyromellitimide). layer and. the m f Outer layer of lqsulafloni in "f 6.An insulated electrical conductor as claimed in claim 1, the twomaterials are in the form of a laminated tape which 18 wherein theinside coat is cross linkc "*Pl mound Conductor and melt 7. An insulatedelectrical component, the insulation of fluol'mifted i y P Py copolymerto E 3 which comprises an inside coat of a thermoplastic polyimidewalled insulation. Wire A ls the product of example i of the selected fthe group consisting f l 1 12- present specification. The numbers givenin the Table indicate dodecamethylene pyromellitimide) and l 1,13. theOrder of rating of the Various Y where the tridecamethylenepyromellitimide) and an outside coat of an difference in results was notsufficient to distinguish between a infusible aromatic i slightdifference in pr perties nd xp ri or p g 6S 8. An insulated electricalcomponent as claimed in claim 7 error, the wires concerned have beenglven equal ratings. A as h i an d i i l outermost coat comprising ablank indicates that the wire concerned could not be tested m d coat f tfl,lz-d d eth l n romelliti ide). (because the insulation had failed a!the temperature 9. An insulated electrical component as claimed in claim7, cerned). wherein the inside coat is an extruded coat.

2. An insulated wire, the insulation comprising two coats ofpoly(1,12-dodecamethylene pyromellitimide) underlying at least two coatsof an infusible aromatic polyimide.
 3. An insulated wire as claimed inclaim 2 having an additional outermost coat comprising pigmentedpoly(1,12-dodecamethylene Pyromellitimide).
 4. A wire as claimed inclaim 3, wherein the aromatic polyimide is poly(4,4''-phenyl etherpyromellitimide).
 5. An insulated wire, the insulation comprising atleast two coats consisting essentially of poly(1,12-dodecamethylenepyromellitimide) underlying at least two coats consisting essentially ofpoly(4,4'' -phenyl ether pyromellitimide).
 6. An insulated electricalconductor as claimed in claim 1, wherein the inside coat iscross-linked.
 7. An insulated electrical component, the insulation ofwhich comprises an inside coat of a thermoplastic polyimide selectedfrom the group consisting of poly (1,12-dodecamethylene pyromellitimide)and poly(1,13-tridecamethylene pyromellitimide) and an outside coat ofan infusible aromatic polyimide.
 8. An insulated electrical component asclaimed in claIm 7 as having an additional outermost coat comprising apigmented coat of poly(1,12-dodecamethylene pyromellitimide).
 9. Aninsulated electrical component as claimed in claim 7, wherein the insidecoat is an extruded coat.